The invention concerns a method for making composite fibre sandwich structures with upper and lower outer layer, at least one sectional core, and with a cut-out, in which the sectional core(s) are shaped and/or cut to size in accordance with the cut-out to be produced and the upper and lower outer layer are cut and cut to size in the region of the cut-out in matching fashion. It further concerns a composite fibre sandwich structure with upper and lower outer layer, at least one sectional core, and with a cut-out, in which the sectional core(s) are shaped and/or cut to size in accordance with the cut-out to be produced and the upper and lower outer layer are cut and cut to size in the region of the cut-out in matching fashion.
At the present time, the use of composite fibre structures with a large area is becoming increasingly important in the aircraft, ship and rail vehicle construction sectors.
As a consequence of their high load-bearing capacity combined with a low structural weight, sandwich structures with a sectional core and a composite fibre construction are often suitable for use here. Otherwise aluminium structures are used as an alternative. Thus, U.S. Pat. No. 3,912,380 proposes a sandwich structure for larger reflector mirrors with a honeycomb structure. The honeycomb structure is to consist of aluminium and the top and bottom are each provided with an appropriate coating to receive the mirror. Precisely in the centre of this structure, intended to form a telescope, a hole or through cut-out is provided, this cut-out being achieved by a simple cutting away of parts of the honeycomb structure and the outer layers.
However, door and/or window cut-outs also always have to be made in this walling for aircraft, ships or railway vehicles. As is well known, these are produced by making cut-outs in the solid material. It is very expensive to produce a durable structure notwithstanding, despite weakening the sandwich structure at this point. To implement an integral method of construction here for the cut-outs which is appropriate to composite fibre structures, reinforcing elements must be inserted where necessary, which is associated with the above-mentioned high expenditure as regards design and fabrication technology.
Therefore, the object of the invention is to propose a method of producing composite fibre sandwich structures with outer layers and sectional core and cut-outs and also such composite fibre sandwich structures themselves, by means of which a sandwich structure is feasible, which is appropriate to the stress and the fabrication method, with cut-outs for windows and doors for example.
The object is achieved in a method as described above in that the the upper and/or lower outer layer are so cut and cut to size, that cut-to-size components remain which initially partly cover the region of the cut-out, strips are laid or draped around the perimeter of the cut-out contour, the cut-to-size components of the upper and lower outer layer are wrapped around the shaped or cut-to-size sectional cores and the strips around the perimeter, appropriate to the composite fibre and the sandwich structure is soaked with resin and cured.
The object is achieved in a composite fibre sandwich structure as described above in that strips are provided which surround the cut-out and are applied to the walls of the sectional cores and in that the lower outer layer and/or the upper outer layer are shaped and/or cut in oversize manner in the area of the cut-out and project into the cut-out and cover the surrounding strips. Further embodiments of the invention are defined in the respective sub-claims.
The problem is solved by a method according to the generic part of claim 1 in that the sectional cores are shaped and/or cut to size as required for the cut-out to be produced, the upper and lower outer layers are cut through and cut to size appropriately in the area of the cut-out, strips are inserted or draped around the perimeter of the contour of the cut-out, the cut-to-size components of the upper and lower layer surround the sectional cores which have been shaped or cut to size and the surrounding strips suitable to the composite fibre construction and the sandwich structure is soaked with resin and cured. The problem is also solved by a composite fibre sandwich structure with upper and lower outer layer, at least one sectional core, and with a cut-out, in which the section cores are shaped and/or cut to size in accordance with the cut-out to be produced and the upper and lower outer layer are adjusted in the area of the cut-out by being cut and cut to size, which is characterised in that strips are provided which surround the cut-out and are applied to the walls of the sectional cores and in that the lower outer layer and/or the upper outer layer are shaped and/or cut in oversize manner in the area of the cut-out and project into the cut-out and cover the surrounding strips.
It is particularly preferable to integrate a frame into the design of the structure direct by using the shaping of the sectional cores, into which a window pane, for example, can be inserted direct and attached with adhesive. Consequently no further design measures are necessary for attaching a window pane of this type in the cut-out.
As a consequence of the advantageous layers of woven or non-woven material comprising the upper and lower outer surfaces, adjusted to the respective loadings occurring in the form of forces applied to the sandwich structure, and the shear elements within the sandwich structure, also in the area of the cut-out, no significant warping arises there. Therefore, a window pane inserted in this cut-out can be held almost without loading from forces acting within the sandwich structure.
Strips made of woven or non-woven fibre materials with a unidirectional orientation, placed in position in the area of the walls of the cut-out or shaped sectional cores, are given particular preference. In this case the strips are laid so that they run round the cut-out in an arrangement which is perpendicular to the outer layers. This transfers any forces in the direction around the cut-out, i.e. remaining in its plane. As a consequence of the advantageous and preferred positioning of the respective outer layers on the outside of these strips of material with a unidirectional orientation, the positive characteristics of the orientation of the fibres in the top and bottom layers can also be utilised in the area of the cut-out. Warping in the area of the support surface of a window pane with the preferred stepping as the shaping of the sectional cores surrounding the cut-out is also restricted advantageously to a very low level in the form of the two outer layers, laid one over the other, thus a double woven or non-woven layer.
The sectional cores are preferably each cut to size in steps in the area of the cut-out in such a way that a support surface is produced for elements to be inserted in the cut-out, such as, for example, window or door elements etc. Here the sectional cores should preferably either be removable silicone cores sheathed with a woven or non-woven fibre material, cut from non-removable, so-called lost foam cores, or pre-shaped as woven hoses in correspondingly pre-shaped master moulds and placed on the lower outer layer as pre-shaped hollow components.
It is particularly preferable to place the sectional cores on the lower outer layer made of woven or non-woven fibre material with an application-specific orientation, then to cut through or cut out the lower outer layer, especially diagonally for a rectangular cut-out, then to place the upper outer layer on the sectional cores and cut through it or cut it out and then wrap the cut areas of the outer layers round appropriately.
It is particularly preferable if the outer layers along the diagonals of the cut-out are cut through and the triangular elements or laps produced in this way are cut to size again in such a way that the area of their respective tips is cut off parallel to the edges of the cut-out with a defined overlap in respect of these edges. This then produces trapezoidal cut-to-size components, which are then laid along the steps of the pre-shaped sectional cores. In this case it is advantageous for the oversizing of the cut-to-size components to be selected in such a way that the outer layers are shaped so that they overlap each other in the support area for the elements to be inserted and are shaped so that they are in contact with the walls of the sectional cores delineating the respective cut-out.
Preferably, after the sandwich structure has been cured, the cores of the sectional cores should be removed from the finished moulded component. However, as an alternative to the latter they can also remain in the structure, for example as foam cores, which can no longer be removed from the cured woven or non-woven layers of the composite fibre material.
It is particularly advantageous for the cut-out to be used as the window or door cut-out in the walling of a railway vehicle or aircraft. In this case the prefabricated window panes are simply inserted and attached with adhesive. No further processing is necessary.
Producing the sheathing surface for the sectional cores determines the size when tailoring the fibre material for components to be cut to size, which will surround the sectional cores in the area of the cut-out. The small sections of the woven layers of the outer layers, which are missing in the corners because of the diagonal cut and subsequent draping on the prefabricated sectional cores, are replaced in the area of the strips inserted perpendicular to the outer layers by the strips as the vertical walling of the cut-out or sectional core in this area. In the area of the supporting surface parallel to the outer layers, in which the two individual layers of the outer layers lie on top of each other, there is also no disadvantage from the missing material in the corners, in a small area in each case, as a consequence of the diagonal cut or subsequent trapezoidal shaping of the cut-to-size pieces of the outer layers. Nevertheless, a good and stable support surface for a window pane, for example, is provided within the cut-out.
The sectional cores are particularly preferable in the form of so-called lost foam cores, if a high thermal and/or sound insulation is required from the sandwich structure. Sound insulation of this type can also be achieved by selecting an appropriate orientation for the fibres in the outer layers. A fibre orientation of xc2x130xc2x0 has proved particularly effective here. The upper and the lower outer layers may consist of different fibre layers, each of which may demonstrate different orientations in their fibres. This enables different advantageous configurations to absorb the forces acting on the sandwich structure simultaneously which therefore no longer have a negative effect on the sandwich structure as such.